1. Field of the Invention
The present invention relates to a process and an apparatus for xe2x80x9cflow solderingxe2x80x9d (which is also referred to as xe2x80x9cwave solderingxe2x80x9d) wherein electronic components are mounted onto (or bonded to) a board (or a substrate) by means of a lead-free solder material.
2. Description of Related Art
In recent years, it has been strongly desired to increase the reliability of an electronic circuit board which is contained in a downsized electronic device while still maintaining and a high performance of the electronic device. Therefore, there is an increasing demand to increase a reliability property such as the thermal shock resistance and the mechanical strength of a connecting portion which is formed by soldering an electronic component onto a board in the field of mounting the electronic components.
Moreover, while concern about the protection of global environment is increased in a worldwide scale, a regulation or legal system to control industrial waste treatments is being arranged. Although an Sn-Pb based solder material, which contains Sn and Pb as its main components (e.g. a so-called xe2x80x9c63 Sn-37 Pbxe2x80x9d eutectic solder material), is generally used in an electronic circuit board which is incorporated in an electronic device, lead contained in such solder material may cause environment pollution if it is subjected to an inadequate waste treatment. As a result, research and developments are carried out as to a solder material which does not contain lead (i.e. a so-called lead-free solder material) as an alternative to a solder material which does contain lead.
A conventional flow soldering process for producing an electronic circuit board by connecting a electronic components to a board such as a printed board as well as an apparatus for such process will be described with reference to drawings below. FIG. 3 shows a schematic view of the conventional flow soldering apparatus.
At first, a board is prepared prior to soldering, wherein through holes are formed through the board, and an electronic component is located thereon by inserting a lead (e.g. an electrode) of the component into the through hole from an upper surface of the board. In such a board, a land which is made of copper or the like is formed on a region (A+B+C, see FIG. 4) consisting of a surface (A) which defines the through holes as well as an upper surface portion (B) and a lower surface portion (C) of the board, where portions (B) and (C) surround the through hole, and such a land is connected to a circuit pattern on the upper surface of the board. On the other hand, regions of the upper surface and the lower surface of the board, except for the lands, are covered by a solder resist.
Next, the board is subjected to a pre-treatment in which the lower surface of the board on which no electronic component is located thereon is applied with flux by means of a spray fluxer (not shown). The pre-treatment is conducted in order to improve wetting and spreading of the solder material on a surface of the land by removing an oxide film (such as a film formed by natural oxidation) which is inevitably formed on the land.
Then, referring to FIG. 3, a thus prepared board (not shown) is put into the flow soldering apparatus 60 while the upper surface on which the electronic component is located is kept upward (with regard to the drawing), and the board is mechanically transferred in a direction of the arrow 61 inside the flow soldering apparatus 60 with a substantially constant velocity by means of a conveyer. In the flow soldering apparatus 60, the board is first heated in a preheating zone by means of a preheating unit (or preheater) 62 in order to make the flux applied to the board, according to the pre-treatment, effectively display its activity ability.
Thereafter, when the board is conveyed into a solder material supplying zone located above solder wave nozzles 64 and 65, the solder material (not shown), which is in a molten state by heating beforehand in a solder material supplying unit 63, is supplied to the board from its lower side through the primary wave nozzle 64 and the secondary wave nozzle 65 in the form of a primary wave and a secondary wave respectively. The solder material thus supplied goes up from the lower surface of the board by means of the capillary action through an annular space between the surface of the through hole (i.e. the land) and the lead which is inserted through the through hole from the upper surface of the board. Thereafter, the solder material naturally cools by releasing its heat to surrounding areas of the board with its natural cooling rate, so that the solder material thereby solidifies to form a connecting portion of the solder material (or a so-called xe2x80x9cfilletxe2x80x9d). In this step of supplying the solder material (or the step of flow soldering), the primary wave functions so as to sufficiently wet the surfaces of the lead and the land with the solder material, and the secondary wave functions so as to remove the solder material on regions covered with a solder resist. As a result, the solder material does not form a bridge by remaining on the board and thereby solidifying between the lands (the bridge is not desirable because it causes a short circuit), and the solder material does not form a cornute projection, thereby controlling (or conditioning) the form of the fillet.
As described above, the fillet (or the connecting portion) made of the solder material is formed to electrically and physically (or mechanically) connect the lead of the electronic component and the land formed in the board. p The fillet made of the solder material as described above is required to have a sufficiently large connecting strength between the lead of the electronic component and the land of the board in order to provide a high reliability of the electronic circuit board. However, referring to FIG. 4, if the electronic circuit board 70 is produced by using the lead-free solder material according to the conventional flow soldering process as described above, the solder material having been wetted and spread on the surface of the land 73 (which is located to cover an inside surface which defines the through hole 72 perforated through the board 71 as well as regions which surround the through hole 72 on the upper side and the lower side of the board 71) partially peels off at an interface between the solder material and the land as indicated by the arrow 80 upon the solidification of the solder material. There thus arises a problem in that the connection between the land 73 and the fillet 74 made of the solder material becomes insufficient and thereby a high connecting strength between the lead 75 and the land 73 cannot be obtained.
Such a phenomenon of the peeling-off of the fillet 74 from the land 73 is generally referred to as a xe2x80x9clift-offxe2x80x9d phenomenon, which frequently occurs when the lead-free solder material is used, although it scarcely occurs when the Sn-Pb based solder material is used. The lift-off phenomenon notably occurs especially in the cases where the lead-free solder material which contains Sn and Bi (such as an Sn-Ag-Bi based material) is used and in the cases where the lead-free solder material for connecting a lead which is plated with an Sn-Pb based material is used.
As a reason for the occurrence of the lift-off, it could be generally considered that the solder material used for the flow soldering and/or a metal material which can elute into the solder material upon soldering (e.g. a plating metal for the lead) forms a weak alloy having a lower melting point than that of the initial solder material and a composition which is different from that of the initial solder material (hereinafter, such an alloy is merely referred to as a xe2x80x9clow-m.p. alloyxe2x80x9d) upon the solidification of the solder material from its molten state.
When the molten solder material is at a high temperature it loses its heat mainly via the lead 75 which comes from the electronic component (not shown). In the solder material (the fillet) 74 which is supplied and which adheres to the board 71, a temperature gradient is formed by a flux of such heat passing through the lead 75. The solidification of the solder material 74 progresses according to its temperature gradient, wherein the lowest temperature portion of the solder material 74 is formed at the top of the solder material 74, which is indicated by the arrow 81, and the highest temperature portion is formed in the vicinity of the interface between the fillet 74 and the land 73, which is a good heat conductor. As a result, the solidification begins at the top of the solder material 74 and ends at the interface between the land 73 and the fillet 74 in due course. Upon solidification, and low-m.p. alloy as described above is distributed (or segregated) in a greater amount in a still molten portion of the solder material which has not yet solidified, such that the low-m.p. alloy is transferred to and concentrated (or segregated) in the molten portion in relation to the progress of the solidification. That is, a segregation phenomenon occurs upon solidification of the solder material, and as a result, the low-m.p. alloy 76 gathers at the interface between the land 73 and the fillet 74 which is where the solidification occurs last. In accordance with this manner of solidification, the fillet 74 solidifies at the top first and attaches to the lead 75. As a result, a tension is generated in the direction of the arrow 82 by a contraction due to the solidification of the solder material, and a tension in the direction of the arrow 83 is generated by the thermal contraction of the board 71. It could be considered that the weak low-m.p. alloy 76 which is enriched in the vicinity of the interface between the land 73 and the fillet 74 as described above can not endure these tensions, and as a result, the peel-off phenomenon is caused at the interface as shown in the direction of the arrow 80.
The present invention has been made to solve the problems described above and to further improve the conventional technique as described above. Accordingly, the present invention aims to provide a flow soldering process for mounting (or bonding) electronic components onto a board by means of a lead-free solder material so as to effectively reduce the occurrence of the lift-off phenomenon, and also provide an apparatus for conducting such a process.
According to one aspect of the present invention, there is provided a process for mounting electronic components onto a board by means of a lead-free solder material (which is also simply referred to as the solder material). The process comprises supplying a melt of the solder material in a solder material supplying zone such that the solder material adheres to a predetermined portion of the board, and thereafter cooling the board by a cooling unit in a cooling zone such that the solder material adhering to the board is quickly cooled (or quenched) so as to be solidified.
In particular, it is sufficient that the cooling of the board (and thus the rapid cooling of the solder material) in the cooling zone of the present invention is carried out by positively (or forcedly) cooling the board by means of the cooling unit for at least a period between a time at which the solder material in the molten condition has just been at its melting point temperature and a time at which all of the solder material (or a connecting portion of the solder material) has just completed its solidification. As far as such cooling is ensured, any appropriate cooling unit may be used for an appropriate time including the above period. Further, the cooling of the board in the cooling zone may be carried out in any appropriate manner as far as the solder material which is adhering to the board is rapidly cooled by cooling the board positively (or forcedly) by means of the cooling unit. For example, the whole board may be cooled, or at least a part of the board (i.e. only a necessary part) may be cooled.
According to the present invention as described-above, the lead-free solder adhering to the board is quickly cooled (or quenched) by positively cooling the board using the cooling unit, so that a period from the start of the solidification of the solder material to the end of the solidification (i.e. the complete solidification) is thereby shortened. In the conventional process, the segregation phenomenon occurs in the solder material by the temperature gradient within the inside of the solder material (or the fillet) since the board is not positively cooled, and as a result, the weak low-m.p. alloy finally gathers in the vicinity of the interface between the fillet and the land as described above. On the other hand, in the present invention, the solder completely solidifies in a shorter period as compared with the conventional process. Accordingly, the segregation phenomenon can be alleviated, and in particular, the gathering of the low-m.p. alloy in the vicinity of the interface is avoidable so that the occurrence of the lift-off phenomenon is thereby suppressed.
Furthermore, in the conventional process, a metal phase which is hard and brittle (such as a Bi phase or a Bi mass) is formed in the fillet upon the solidification of the solder material and thereby provides brittleness to the fillet, and as a result, the fillet has a low mechanical strength. According to the present invention, on the other hand, the period required for the solidification of the solder material is shortened, and thus, the period during which such a metal phase grows is shortened, which makes a structure of the metal phase in the fillet minute (or fine). As a result, the mechanical strength of the fillet is increased by the present invention.
In a preferred embodiment, the lead-free solder material is cooled in the cooling zone at a cooling rate of at least 200xc2x0 C./min. The cooling rate is preferably as large as possible, and the cooling rate is, for example, in a range of about 200 to 500xc2x0 C./min, and preferably about 300 to 500xc2x0 C./min.
It is noted that the cooling rate is referred to as a temperature decreasing rate corresponding to decreasing the temperature of a solder material portion (and in particular, a portion to be a connecting portion (or fillet portion) made of the solder material by solidification of the solder material), and it is inherently an average temperature decreasing rate over a period between a time at which the solder material in the molten condition has just been at its melting point temperature and a time at which the solder material (or the whole solder material portion) has just completed solidifying. However, there is practically no problem when such an average temperature descreasing rate is regarded as an average of a measured temperature of the solder material portion at a moment when the board arrives into the cooling zone and a measured temperature of the solder material portion at a moment when the board departs out of the cooling zone, and thus, such an average of the measured temperatures is conveniently used as the average temperature decreasing rate (i.e. the cooling rate) in the present specification. The temperature of the solder material portion (i.e. the connecting portion of the solder material) can be measured by a thermocouple placed on (for example, attached to) a predetermined portion of the board on which the solder material is to adhere (for example, a land located on one side of the board to which the solder material is supplied), and data from the thermocouple can be recorded together with time.
The positive cooling of the board in the cooling zone as described above can be carried out by gas cooling or by liquid cooling, and as the cooling unit for cooling the board, a unit which employs the gas cooling or the liquid cooling can be used. It is noted that gas cooling means the operation of cooling the board by using a gas (such as air (or ambient air), or an inert gas such as nitrogen gas) as a coolant whose temperature is lower than the temperature of the board (and, in particular, lower than the temperature of the solder material which is adhering to the board), and is generally considerably lower than such temperature. The gas cooling allows the board to be in contact with such gas so as to cool the board. For example, such gas has a temperature in the range between xe2x88x9220 and 30xc2x0 C., and preferably in the range between xe2x88x9210 and 10xc2x0 C. The gas cooling can be carried out by passing the board through an atmosphere including such gas or by blowing such gas directly toward the board. Also, liquid cooling means the operation of cooling the board by using a liquid (such as water, liquefied nitrogen or other liquid) as a coolant whose temperature is lower than a temperature of the board (and, in particular, the temperature of the solder material which is adhering to the board), and is generally considerably lower than such temperature. The liquid cooling allows at least a necessary portion or the whole board to be in contact with such liquid so as to cool the board. For example, such liquid has a temperature in the range between 1 and 30xc2x0 C., and preferably in the range between 1 and 10xc2x0 C. The liquid cooling can be carried out by dipping the board into an amount of the liquid or by spraying the liquid toward the board. The liquid cooling can use a latent heat of vaporization of the liquid, which is advantageous for the readily rapid cooling.
Concretely, as the cooling unit using the gas cooling, a unit may be used which comprises a blower or a gas flowing device (including, for example, a pump) which supplies the low temperature gas into the cooling zone, and then discharges the gas from the cooling zone. Such supplied gas forms a low temperature atmosphere through which the board is passed. Further, a unit comprising at least one nozzle, a fan, a spot cooler or the like which blows the low temperature gas directly toward the board may also be used. As the cooling unit using the liquid cooling, a unit may be used which comprises either a bath containing the liquid into which the board is dipped so as to cool an entire portion of the board, an atomizer which forms and sprays a mist of the liquid toward at least a portion of the board, a nozzle which supplies a relatively small amount of the liquid to at least a portion if the board or the like. Depending on the kind of such cooling unit, it may be located in the cooling chamber, or the unit may be connected to the cooling chamber so that a portion or the whole unit thereof is located outside the cooling chamber. It is of course possible to employ any combination of the above-mentioned various kinds of the cooling units. In order to obtain a larger cooling rate, it is preferable that the liquid cooling rather than the gas cooling is employed for the purpose of the positive cooling.
In the case of the gas cooling, the cooling rate can be obtained as desired by controlling the temperature of the gas which is supplied to the cooling zone and the flow rate of such gas, a transfer speed of the board and so on while considering the heat capacities of the board and the components which are located on the board. In the case of liquid cooling, a desired cooling temperature can be obtained by controlling a temperature of the liquid which is contacted with the board, the transfer speed of the board, and so on while considering the heat capacities of the board and the components which are located on the board. For instance, a cooling rate of about 210xc2x0 C./min. is obtained by passing a board made of a glass epoxy resin having a size of 200 mmxc3x97200 mmxc3x970.8 mm at a speed of about 1.2 m/min, through a chamber having a size of 1000 mmxc3x97500 mmxc3x97500 mm while blowing a gaseous atmosphere having a temperature of about 25xc2x0 C. towards the board at a flow rate of about 2 liter/min.
The positive cooling of the board in the cooling zone is preferably conducted by the gas cooling in which the cooling unit uses nitrogen gas. In such a case of gas cooling using nitrogen gas, oxidation of the solder material can be prevented so that a wetting property of the solder material (and especially the wetting property with respect to the land) is improved, which thereby further decreases the occurrence of the lift-off.
In another preferred embodiment, the process of the present invention further comprises, after supplying the molten solder material to the board in the solder supplying zone so as to adhere to the predetermined portion of the board and before cooling the board in the cooling zone, locating the board in a conditioning zone having an atmosphere at a temperature which ensures a complete molten state of the solder material adhering to the board. It is sufficient that the complete molten state of the solder material is ensured at least at a time just before the board is cooled in the cooling zone. It is readily understood by those skilled in the art that such a conditioning zone is located downstream of the solder material supplying zone and upstream of the cooling zone with respect to the transfer direction of the board.
The conditioning zone functions to condition the board after the solder material is attached to the board and before the board is rapidly cooled. In particular, it conditions the board (particularly the solder material adhering to the board) such that the solder material at any position of the board is substantially entirely in the molten state (that is, the entire solder material is substantially and homogeneously in the molten state over an entire portion of the board to which the solder material is to be adhered thereto). Concretely, when the board comes into the conditioning zone while a portion of the solder material which is adhering to the board is naturally cooled and begins its solidification, the board (and thus the solder material) is heated in the conditioning zone so that the entire portion of the solder material which is naturally solidifying is again melted preferably completely before the board is cooled in the cooling zone. When the board comes into the conditioning zone while the entire portion of the solder material which is adhering to the board is substantially in the molten condition, the board is located in an atmosphere of the conditioning zone in which the board (and thus the solder material) does not lose an amount of heat excessively (preferably be being supplied with heat) so that the solder material does not start its solidification at least before the board goes into the cooling zone, and thus, the molten state of all of the solder material can be kept. Thus, in this case, no positive heating of the board is necessarily carried out. The conditioning zone may be referred to as a heating zone which heats the board by supplying an amount of the heat to the board so as to raise the temperature of the solder material, or as a constant temperature zone which is kept at a certain temperature and in which an excessive temperature drop (and thus excessive cooling) of the solder material (thus the board) is suppressed during when the board passes through the conditioning zone (or until it comes into the cooling zone) by supplying a lesser amount of heat or by thermally insulating the conditioning chamber.
The provision of the board in the conditioning zone as described above can reduce a temperature difference (or narrow a temperature distribution) within the solder material of each connecting portion as well as a temperature difference within connecting portions of the solder material over the board as a whole. In other words, the conditioning zone functions to make the board and thus the solder material in a thermally uniform condition. In the absence of the conditioning zone, the solder material may begin to partly solidify before the board is cooled in the cooling zone, and in such a case, an initial temperature upon which rapid cooling begins in the cooling zone may vary between a single connecting portion of the soldering material or throughout all of the connecting portions to some extend. On the other hand, when the board is conditioned to be thermally uniform beforehand by providing such a conditioning zone upstream of the cooling zone, any such variation of the initial temperature upon which rapid cooling begins, which thereby results in a variation of the period required for the solidification of the solder material, can be suppressed. As a result, it is possible to further decrease the occurrence ratio of the lift-off.
The temperature of the atmosphere of the conditioning zone as described above can be any temperature as long as it both ensures the molten condition of the solder material before the board goes into the cooling zone and it is less than a heat resistant temperature of the electronic component mounted on the board so as to avoid thermal damage of the electronic component. The temperature of the conditioning zone is preferably in the range which is not less than the melting point of the solder material and which is less than the heat resistant temperature of the electronic component in order to completely or entirely melt the solder material. More preferably, the temperature of the conditioning zone is within the range which is higher than the melting point of the solder material by 10xc2x0 C. and lower than the heat resistant temperature of the electronic component by 5xc2x0 C. However, the present invention is not limited to the above. For example, where the solder material which is adhering to the board is at a higher temperature than the melting point of the solder material (i.e. the solder material has not started its solidification) when the board comes into the conditioning zone and the conditioning zone can prevent such excessive temperature decreases of the solder material therein such that the solder material starts to solidify when the board passes through the conditioning zone, the temperature of the conditioning zone is not necessarily higher than the melting point of the solder material. It is of course preferable even in such a case that the temperature of the conditioning zone is not lower than the melting point of the solder material and lower than the heat resistant temperature of the electronic component from a viewpoint of the complete molten condition of the solder material.
It is preferable that the atmosphere of the conditioning zone consists essentially of nitrogen gas. As a result, the oxidation of the solder material and the land is prevented to avoid degradation of the wetting property of the solder material, and as a result, a connecting area of the land with the solder material is sufficiently ensured to suppress the peel-off of the solder material.
The atmospheres of the conditioning zone and the cooling zone can be selected independently of each other, although both of them are preferably of nitrogen gas.
According to another aspect of the present invention, there is provided an apparatus for mounting (or bonding) electronic components onto a board by a flow soldering process using a lead-free solder material (which is also referred to as merely xe2x80x9csolder materialxe2x80x9d as described above). The apparatus comprises a solder material supplying unit (or a solder material supplier) which supplies a molten solder material located in a solder material supplying chamber so as to attach molten solder material to a predetermined portion of a board, located downstream of the solder material supplying chamber, wherein the board is cooled by a cooling unit (or a cooler) so as to rapidly cool and solidify the solder material which is adhering to the board.
In a preferred embodiment of the above-described apparatus, the board is cooled by the cooling unit in the cooling chamber such that the lead-free solder material is rapidly cooled at a cooling rate which is not less than 200xc2x0 C./min. As the cooling unit, a unit may be used which uses the gas cooling or the liquid cooling, and preferably the gas cooling uses nitrogen gas as the cooling gas as explained above with reference to the flow soldering process according to the present invention.
In another preferred embodiment of the above-described apparatus, the apparatus of the present invention further comprises a conditioning chamber located between the solder material supplying chamber and the cooling chamber with respect to the transfer direction of the board. An atmosphere in the conditioning chamber has a temperature at which the solder material is ensured to be in a completely molten condition. The temperature of such an atmosphere in the conditioning chamber is preferably not lower than the melting point of the solder material and lower than the heat resistant temperature of the electronic component. Further, the conditioning chamber preferably contains a nitrogen gas atmosphere.
It is noted that the term xe2x80x9cchamberxe2x80x9d is intended to mean a structural member which defines a space, the term xe2x80x9czonexe2x80x9d is intended to mean a space (or a spatial member) within the chamber, and the term xe2x80x9catmospherexe2x80x9d is intended to mean a gaseous atmosphere (or a gas) in the space formed by the chamber (thus, an atmosphere in a zone is substantially the same as an atmosphere in a chamber). For example, the xe2x80x9csolder material supplying chamberxe2x80x9d defines the xe2x80x9csolder supplying zonexe2x80x9d of the flow soldering process according to the present invention, and this is similarly applicable to the xe2x80x9ccooling chamberxe2x80x9d and the xe2x80x9cconditioning chamberxe2x80x9d.
The flow soldering process according to the present invention as described above is conveniently carried out using the apparatus according to the present invention. Therefore, it is understood by those skilled in the art that the above descriptions as to the solder material supplying zone and the cooling zone as well as the optional conditioning zone with reference to the preferred embodiments of the process according to the present invention are also applicable to the solder material supplying chamber and the cooling chamber as well as the optional conditioning chamber.
In the flow soldering apparatus according to the present invention, the solder material supplying chamber does not have to be formed such that the solder material supplying zone is definitely divided from other spaces in the apparatus. The cooling chamber may definitely divide the solder material supplying zone from other spaces having relatively high temperatures at least to such an extent that the positive cooling of the board is effectively carried out in the apparatus of the present invention. Also, the conditioning chamber may be such that its atmosphere is definitely divided from other atmospheres having relatively high temperatures (such as an atmosphere of the solder material supplying chamber), but not necessarily definitely divided from other spaces in the apparatus. However, the conditioning chamber and the cooling chamber are preferably structured such that atmospheres of these chambers are divided from each other to some extent from a viewpoint of the thermal efficiency.
It should be noted that the solder material supplying chamber, the cooling chamber and the optional conditioning chamber which are described above with reference to the flow soldering apparatus according to the present invention do not necessarily have to be used upon conducting the flow soldering process according to the present invention, and other apparatus may be used so long as it allows the rapid cooling as described above.
The lead-free solder material which can be used for the flow soldering process and/or the flow soldering apparatus according to the present invention includes, for example, an Sn-Cu based material, an Sn-Ag-Cu based material, an Sn-Ag based material, and Sn-Ag-Bi based material, an Sn-Ag-Bi-Cu based material or the like. With regarding to the board, a board made of, for example, a paper phenol, a glass epoxy resin, a polyamide film, a ceramic or the like can be used. The electronic component connected to the board can be any electronic component which is connected to the board by means of the through hole formed through the board. For example, the electric components can be a DIP IC (Dual In-line Package-Integrated Circuit), a connector and an axial lead component. However, these are described only for illustrative purposes, and the present invention is not limited thereto.